Improving 5.1 sound from my htpc?

[darsunt]

My htpc has onboard sound chipset. From the green outlet port I've plugged it into an old Sony 250W integrated amplifier, set the realtek software to 5.1 sound, and run it out to my old sony speakers. The sound is much better than the big screen speakers, but kind of odd, like echoes in a big room.
Which components should I replace first to improve sound? Should I get a dedicated sound card? An amp with an S/PDIF interface?

 

[darsunt]

Also the performance of different speakers is uneven. For example, one rear speakers sounds okay while the other sputters. Maybe the stereo wires are too old?